I don't think DOS ever wrote zeroes when extending a file (except when using the FCB function "write random with zero fill"). On a single-user system, you'd only get back your own deleted data, so this at least can't be considered a security problem.

It definitely is a problem if a network server extends files that way!

NTFS has a "zero fill" flag that the OS sets for newly allocated blocks, so it doesn't actually have to fill them on disk - it can just return zeroes when an application tries to read from a part of the file that hasn't been written yet. I believe ext4 and all other modern filesystems have this optimization as well.